1. Field of the Disclosure
The present disclosure relates to gyroscopes, and in particular to disc resonator gyroscopes. More particularly, this invention relates to tuning disc resonator gyroscopes.
2. Description of the Related Art
Mechanical gyroscopes are used to determine direction of a moving platform based upon the sensed inertial reaction of an internally moving proof mass. A typical electromechanical gyroscope comprises a suspended proof mass, gyroscope case, pickoffs, or sensors, torquers, or actuators and readout electronics. The inertial proof mass is internally suspended from the gyroscope case that is rigidly mounted to the platform and communicates the inertial motion of the platform while otherwise isolating the proof mass from external disturbances. The pickoffs to sense the internal motion of the proof mass, the torquers to maintain or adjust this motion and the readout electronics that must be in close proximity to the proof mass are internally mounted to the case which also provides the electrical feedthrough connections to the platform electronics and power supply. The case also provides a standard mechanical interface to attach and align the gyroscope with the vehicle platform. In various forms gyroscopes are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
Older conventional mechanical gyroscopes were very heavy mechanisms by current standards, employing relatively large spinning masses. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as mechanical vibratory gyroscopes.
Spacecraft generally depend on inertial rate sensing equipment to supplement attitude control. Currently this is often performed with expensive conventional spinning mass gyros (e.g., a Kearfott inertial reference unit) or conventionally-machined vibratory gyroscopes (e.g. a Litton hemispherical resonator gyroscope inertial reference unit). However, both of these are very expensive, large and heavy.
In addition, although some prior symmetric vibratory gyroscopes have been produced, their vibratory momentum is transferred through the case directly to the vehicle platform. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation.
In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and the vibrating thin ring gyroscope, are known to have some desirable isolation and vibrational symmetry attributes. However, these designs are not suitable for or have significant limitations with thin planar silicon microfabrication. The hemispherical resonator employs the extensive cylindrical sides of the hemisphere for sensitive electrostatic sensors and effective actuators. However its high aspect ratio and 3D curved geometry is unsuitable for inexpensive thin planar silicon microfabrication. The thin ring gyroscope (e.g., U.S. Pat. No. 6,282,958, which is incorporated by reference herein) while suitable for planar silicon microfabrication, lacks electrostatic sensors and actuators that take advantage of the extensive planar area of the device. Moreover, the case for this gyroscope is not of the same material as the resonator proof mass so that the alignment of the pickoffs and torquers relative to the resonator proof mass change with temperature, resulting in gyroscope drift.
More recently, a novel resonator structure comprising a disc using embedded electrostatic electrodes for excitation and sensing has been developed for microgyro applications. In general, the electrodes are formed along with the resonator by through etching a wafer selectively bonded to a baseplate such that the through-etched sidewalls form the capacitive gaps between the electrodes and the resonator and the electrodes and the resonator remain separately bonded to the baseplate. Three-dimensional vibration modes of the disc are excited and sensed to measure angular rate. Some development of such disc resonator gyroscopes (DRG) and applications has already occurred. For a detailed description of a current exemplary DRG assembly, see e.g. U.S. Pat. No. 7,040,163 and U.S. patent application Ser. No. 11/757,395, filed Jun. 4, 2007 and entitled “PLANAR RESONATOR GYROSCOPE CENTRAL DIE ATTACHMENT” by A. D. Challoner, which are incorporated by reference herein.
Performance and operation of such DRGs is facilitated through a variety of tuning techniques. The disc resonators may be physically tuned through precise selective mass removal on the disc resonator structure. In addition, electrostatic tuning of the disc resonators may accomplished by applying appropriate electrostatic tuning signals to bias electrodes of the DRG as it is operated in order to reduce the effect of asymmetry of the disc resonator giving rise to accuracy errors of the inertial rate measurements as well as drift. Previous tuning techniques for DRGs involved multi-step trial and error or computerized search methods such as simulated annealing, which are laborious or time consuming.
In view of the foregoing, there is a need in the art for techniques applied to DRGs to tune them more efficiently. There is particular need for such techniques to tune DRGs early in a manufacturing screening process more easily and at a reduced cost. There is a need for such DRG tuning techniques to lower noise and drift and reduce manufacturing cost of tuning. As detailed below, the present invention satisfies all these and other needs.